What Is the Autumnal Equinox?

As the colors of the landscape begin to change and days and nights become cooler, the autumnal equinox marks the astronomical start of autumn in the Northern Hemisphere. Falling on September 22 in both 2017 and 2018, the equinox is a cause for celebration and reflection for many cultures.

The autumnal equinox marks one of the two points in each year when the sun is positioned directly above the equator, shining equally on the Northern and Southern Hemispheres. The length of day and night on the day of the autumnal equinox are almost, but not completely, equal. The other day on which the sun is positioned this way is called the spring equinox, as it marks the astrological first day of spring in the Northern Hemisphere. The autumnal equinox in the Northern Hemisphere also marks the spring equinox in the Southern Hemisphere.

In the days leading up to the autumnal equinox, daylight hours in the Northern Hemisphere make up more than half of each 24 hour cycle. After the autumnal equinox, nights begin to lengthen and temperatures start to drop in the Northern Hemisphere. This continues until the Winter Solstice in December marks the date with the fewest daylight hours annually.

Surprisingly, day and night are not exactly equal lengths on the date of the equinox. While the center of the sun does set exactly 12 hours after it rises on this day, we begin to experience daylight when the upper edge of the sun crests the horizon, and full dark doesn’t occur until the entire sun has set. In fact, the earth’s atmosphere refracts light from the sun over the horizon for a short time after the sun has set. The result of this is that while day and night are technically exactly equal on the equinox, we experience day as being slightly longer than night.

Interestingly, the autumnal equinox is not the only way to measure the beginning of autumn. As measured by the meteorological calendar, autumn begins on September 1 and ends on November 30. This calendar chooses to split up the year into four three-month segments, with autumn lasting from September through November, winter lasting from December through February, spring lasting from the beginning of March through the end of May, and summer taking up the months of June, July, and August. Meteorological seasons are split up based on climate trends throughout the year, and make it easier to analyse and compare seasonal weather and temperature data.

The autumnal equinox is considered to be a day of thanksgiving and celebration for some pagan groups. It is known in pagan mythology as Mabon, Second Harvest, or Alban Elfed. Neo-druids and pagans travel to sites known to be ancient or regarded as sacred, such as Stonehenge, to recognize and celebrate the autumnal equinox. Related to the equinox is Michaelmas, a Catholic feast traditionally celebrated on September 29th to honor the archangel Michael. Michaelmas was introduced in the fifth century as a replacement for pagan celebrations of autumn.

Other autumnal equinox celebrations happen all over the world. In China the Mid-Autumn Festival, also known as the Moon Festival, is celebrated in late September. The Mid-Autumn Festival celebrates the completion of the harvest season. Traditionally, mooncakes filled with dried fruit, sesame seeds, or lotus are eaten at the festival. Also around the time of the autumnal equinox Japan holds a week of Buddhist services named Higan. The word Higan means “other shore,” and the week is centered around honoring and remembering the spirits of the dead. Japan holds a similar week of prayer during the spring equinox.